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Old 03-11-2003, 12:50 AM
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Post Rear End for Daily Driver

OK guys, here I'm asking about rear ends again :o

I sort of walked away from the 4th generation combo because I don't think I was ready for finding things out the hard way. Anyway, I was thinking 3.43 Posi would be fun, but because I'm used to the 2.73, I'm afraid I'll sacrifice mileage.

This '86 Firebird is my daily driver, and I'm not out to make her a track star. What is the tallest ratio I could plug into my Bird with a Posi or limited slip? Is something in the area of 3.03 feasible or worth the change from a 2.73, and is available in a GM Posi? I believe I can go to a 3.23 or thereabouts, but I'm thinking the taller, the better mileage.

I'm not too familiar with rear ends, so I'm looking for some education here. (PS - I'll prepare myself for "getting it in the rear" with the sophomoric stuff!) <img src="graemlins/boxing.gif" border="0" alt="[boxing]" />

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Old 03-11-2003, 02:01 AM
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Kaylah,
I am sure someone else will have a better (more accurate) answer for ya, but I had an old regal sport coup (see photo album) and I had the 3.08 gears out back (along with the 70 Buick 350 from my Skylark!) That thing was simply amazing! Didn't get that bad of gas mileage and it was quite the power house. On stock tires, was running high 14's on street tires. (Couldn't hook up at the local track... it was rather comical though!)
I always refer to that car as "silly" because it was just that, any time you nailed the gas, you were gonna spin! It was a fun little ride, I wish I hadn't sold the body.

I don't even think 3.23's would be bad either, but I am not a rear end expert. (Boy this topic is just askin for wise cracks - no pun intended!)

Rock on...

Senor
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Old 03-11-2003, 11:26 AM
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If you give me your tire diameter, from bottom to top and the speed that you want to be going at a given RPM I will calculate the ratio for you.

You don't want to cruise at too many RPM's, it doesn't leave much room before floating the valves.

Ron
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Old 03-11-2003, 11:29 AM
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Kaylah, what kind of transmission do you have?

It makes a difference when calculating the ratio, the transmission has a ratio too.

Ron
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Old 03-11-2003, 12:24 PM
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[quote] from Kaylah's photo album

305 C.I.D. Chevrolet LG4 carbuerated Rochester 4bbl (soon to be replaced). Trans is a 700R4 auto, and a single wheelin' 2.73 tailpiece <hr></blockquote>

Hi Kaylah,

You would benefit off the line with a posi unit. And gain needed at the wheel torque from a lower gear (higher #).

But, for cruising, if you put to much gear in it, it may become exactly what you describe as what you don't want.

If you add to little gear to it, you may find that the cost of upgrading wasn't effective for performance gains versus actual expense.

I just went from a 2.73 gear ratio posi rear end to a 3.42 posi rear with my 79 Malibu that has a fairly massaged SBC 350 and 350T transmission in it. The gains for performance was great, I really like it, but, at that gear ratio I gained a ton of rpms (from 2200 rpm to 3000 rpm @ 65 mph) on the highway and have lost a bunch of gas mileage in the process (from 14 mpg hwy to 10 or 11 mpg hwy).

Your Firebird will fair a bit better with your 700R4 trans and it's overdrive for gas mileage and highway rpms.

There's always a give or take when it come to performance.

Have fun, good luck.
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Old 03-11-2003, 01:13 PM
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I run 3:23s in my Willys and that seems a good compromise between mileage and performance. I run 3:54s in my '53 Chevy pickup and I don't like the cruise RPM but am switching to a 200R4 OD tranny so will probably like the 3:54s now. Anything between 3:23 and 3:54 is good for a stop light bandit.
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Old 03-11-2003, 02:33 PM
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I'm not sure what kind of mileage to expect, but I know that some Camaros came with a limited slip 3.73 in '86. I have one from an '86 Camaro that had the 2.8 60 degree v6 and the 5-speed tranny.
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Old 03-11-2003, 06:36 PM
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hey folks, this is my first post and I had a question regarding this same type of stuff. I have a 1977 Camaro with a 305 and Muncie 4 spd from an older car. I have 2.73 gears since this was a convereted automatic. I drive this car mostly in town, but enjoy cruising. I want to improve acceleration, but save some mileage. Should I shoot for something between 3.21 and 3.42 as mentioned above or could I manage 3.55's? Any help is appreciated. Now all I have to do is replace the u-joints on my driveshaft, which decided to fall out in town today.
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Old 03-11-2003, 07:20 PM
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OK..I will be the Devil's advocate here.

I believe you can hurt mileage with a too high of a ratio(2.73/3.08). I have a 3.50 ratio in my 9" Ford equipped 32 Ford with a ZZZ 350-345HP engine. 400 Turbo with 28" tall tires. 2940 RPMS at 70 MPH.

My friend has a 3.89 ratio in his 9" Ford equipped 32 Ford coupe, ZZ4 350-355HP engine. Automatic OD (which makes his overall ratio 2.72) and 29" tires. 2200 RPMS at 70 MPH

He turns over 700 RPMS less at 70 MPH but I get 2-3 mpg better mileage than he does. Very similar cars, power and equipment. He is NOT in the cruise RPM range of the camshaft and I am.

My 1st gear is 2.48
2.48 X 3.50 is 8.68 first overall

His 1st gear is 2.78
2.78 X 3.89 is 10.81 first overall

His 32 should be quicker than my 32 but it is not. Too much gear, it spins while my car hooks better, not too much gear.

I would go with the 3.43 because most Muncies are only 2.20 in 1st gear. The wide ratio is 2.54.


The correct gear can make or break a combination.




[ March 11, 2003: Message edited by: Deuce Roadster ]</p>
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Old 03-11-2003, 07:40 PM
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kool thanks duece, I meant to metnion that me and a friend were discussing that very idea and agree completely. We think to high a gear can bog the engine making it less efficient and wasting more gas.
I definately believe that 3.43's are the way to go for my setup. Thanks for the reassurance.
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Old 03-12-2003, 12:07 AM
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Thanks for all the good discussion. I am leaning toward the 3.23 Posi idea, but I was wondering about the 3.0 range because I do a lot of highway driving sometimes at 70-80 on open road, and I'd like to keep the RPM's on the lower side.

I'm having a set of Michelin Rainforce 205/65R/15's put on that I got at a good deal. But on the next set, I am leaning toward increasing the size and going with a 245/55 or 60R/16. But I don't want to go too thin, so I'll probably use a 60 series (I can't stand the way those skinny 45's and 50's look). I'm not sure of the actual diameters, but those sizes should give a range. After they get mounted tomorrow, I can get the actual measurement.

And yes, the trans is a 700R4 that I will likely upgrade using a mild performance rebuild when that time comes.

Thanks again gentlemen, this is a good discussion. I have these hunches and feelings about how things should play out, but have oh so few folks to bounce the ideas around with.
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Old 03-12-2003, 06:13 AM
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Kayla, just wanted to save you the measurement of the tire. The 205-65-15 comes out to be 25.49 inches in dia. With the 3.23 and the 700 see the chart below.

RPM 1st 2nd 3rd 4th
1000 8 14 23 33
1500 11 21 35 49
2000 15 28 46 66
2500 19 35 58 82
3000 23 42 69 99
3500 36 49 81 115
4000 30 56 92 131
4500 34 64 104 148
5000 38 71 115 164
5500 41 78 127 181
6000 45 85 138 197
6500 49 92 150 214
7000 53 99 161 230

These are just about the same for the turbo 350 except the 4th gear shift. I'm in desperate need of a 700R. That 4th gear in your 700R is going to keep the RPM's down sufficiently. You should be fine with the RPM@2500 for 82MPH. I sure wish I had that for the highway. Above 180 you'll probably bend a valve.
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Old 03-12-2003, 06:21 AM
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27.5 is the diameter of the 245-60's. You will be increasing your speed about 5mph at the same RPM's with these tires. 87mph@2500rpm. Just to let you know the advantage of the 16".

Ron
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Old 03-12-2003, 06:40 AM
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Not disagree with anyone here but tire specs vary a good bit from 1 company to another. As much as 1 inch for the same 235/70/15 tire.


Sad but true........


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Old 03-12-2003, 08:19 AM
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Thanks again Ron.

You've confirmed my thoughts about the 16's. I realize that actual sizes will differ from mfgr to mfgr. But it does give me a set of working figures.

I also like the range in first and second. My second is nice and steep, so I also see a good little bit of get-up-and-go . I think with all of your help, and the others posting on this thread, I can make some good future plans.

Tahnks again everyone
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